A centrifugal compressor which is used in a compressor portion or the like of a vehicular turbocharger or a marine turbocharger imparts kinetic energy to a fluid via rotations of an impeller and increases pressure due to centrifugal force by discharging the fluid outward in a radial direction.
Such centrifugal compressors are required to have a high pressure ratio and high efficiency over a wide operating range. Accordingly, various concepts have been devised and implemented for scroll structures.
As prior art, for example, Patent Document 1 (Japanese Patent No. 4492045) describes a technique with respect to a centrifugal compressor comprising a casing provided with a spirally formed scroll flow path, wherein the scroll flow path is formed such that a flow path width in an axial direction gradually increases from inward to outward in a radial direction and the flow path width is maximum on an outer side in the radial direction of an intermediate point of the flow path width in the radial direction.
In addition, Patent Document 2 (Japanese Translation of PCT Application No. 2010-529358) describes a centrifugal compressor for a turbocharger, wherein the centrifugal compressor comprises a spiral housing and a diffuser, and the diffuser is formed with an enlarged diameter so as to reduce a negative pressure range in a transitional region or a region in which a tongue portion is positioned in the spiral housing.
Patent Document 1: Japanese Patent No. 4492045
Patent Document 2: Japanese Translation of PCT Application No. 2010-529358
Although improvements of a cross-sectional shape of a scroll flow path such as that described in Patent Document 1 and improvements of a diffuser portion such as that described in Patent Document 2 have been made, further improvements are required to enhance compressor efficiency.
As shown in FIGS. 12 and 13, a diffuser 02 is formed on an outer circumferential side of an impeller 01 of a compressor and a scroll flow path 03 is provided on an outer circumferential side of the diffuser 02. A cross-sectional shape of the scroll flow path 03 is generally formed in a circular shape, and a flow path connection 04 at a winding start and a winding end of the scroll flow path 03 is connected at a tongue portion 05. In addition, discharge subsequent to the winding end is to be performed through an outlet flow path 06.
FIG. 13 shows, on top of each other, scroll cross-sectional shapes taken at angles θ1, θ2, . . . which occur at intervals of a predetermined angle Δθ in a clockwise direction from the tongue portion 05.
At the tongue portion 05, as indicated by the hatched lines in FIG. 13, the flow path connection 04 is shaped such that a circular portion 09 is connected to an outlet portion 011 of the diffuser 02 that is tangent to the circular portion 09.
In addition, in a vicinity of the tongue portion 05, there is a problem that a separated flow is created due to interference between a diffuser outlet flow A and a scroll flow path internal spiral flow B, which results in flow loss. The interference between the diffuser outlet flow A and the scroll flow path internal spiral flow B will now be described with reference to FIG. 9B. FIG. 9B is a sectional view taken along line C-C in FIG. 12, in which the outlet flow path 06 with a circular cross-sectional shape and the scroll flow path 03 with a circular cross-sectional shape intersect with each other to create a ridge line P at an intersection in the vicinity of the tongue portion 05. Therefore, the diffuser outlet flow A has an upward velocity component in the vicinity of the tongue portion 05 and interferes with the scroll flow path internal spiral flow B. Due to the interference, a separation of flow is created in the vicinity of the tongue portion 05 and causes flow loss.